The techniques for securing studs to various types of base metal, such as for example planting the window frame fixing T-studs to the vehicle body, are known and widely employed in the industry, and heretofore, arc welding has been popularly used as such stud planting means by utilizing both base metal and stud as electrodes of opposite polarities.
However, it has become known that the hitherto used stud welding techniques involve various problems, particularly concerning metal depositability and corrosion resistance.
The problem of metal depositability is mostly associated with the physical conditions of the welding operation, while corrosion resistance has been considered primarily attributable to ageing and other chemical factors after welding, but recent studies have brought to light the fact that the physico-chemical conditions at the time of welding are a decisive factor for corrosion resistance.
As is well known, stud welding is generally practiced by conducting a current of a density corresponding to the stud diameter, but in such welding operation, atmospheric air is taken in for various reasons to form voids that affect metal deposit. In order to overcome such problems, some noval arc shielding techniques have been devised recently. For instance, Japanese Patent Publication No. 20033/75 proposes use of an inert gas singly or in admixture with other gas as an arc shield. Generally, carbon dioxide gas argon gas or such is employed as the shielding gas, and such low-current stud welding method which is capable of providing high mechanical strength and good metal depositability is finding wider and wider applications.
However, even when T-stud welding is performed by using a stored arc stud welder with an improved gas shield, there is inevitably induced deposition of black powder which tends to cause corrosion later, and an analysis of such black powder deposit has revealed that it is a kind of oxide film.
In the case of carbon dioxide gas shield, such oxide film is formed on the base metal 1 as a black powder deposit 3 around the stud 2 as shown in FIG. 1. One of the causes thereof is that the molten drops of arc in the carbon dioxide shielding gas are formed into globules and the so-called globule migration would take place to increase the spatters, resulting in formation of the oxide film in the form of a compound or a mixture. Thus, in this case, there is as much chance of forming an oxide film as in the case of the flash welding where no shield gas is used.
Because of high heat conductivity of carbon dioxide gas, the arc core is large and weld penetration is deep, hence wide bead span and enlarged oxidation area.
In case of using argon gas for arc shielding, the arc core is small and also the bead span is reduced because of low heat conductivity of argon gas, so that although the metal depositability is worsened, the oxidation area is lessened, and hence it looks as if no black powder deposition takes place in this case, but a salt spray test evidently shows that a ring of black powder 4 is formed on the base metal 1 around the stud 2 with a certain spacing therefrom as shown in FIG. 2, thus involving the possibility of forming rust as in the case of said carbon dioxide gas shield.
This is for the reasons that, in the case of argon gas, spray-like impact is given to the base metal by the free ions to let the compound or mixture molecules or particles fly and drift in the area of a given radius and thereby get densified.
Therefore, in use of the conventional stud welding techiques employing a shield gas, there has been taken only individual measures according to the type of the shield gas used, and consideration has been given only to the chemical characteristics of such shield gas.
Accordingly, there was necessitated a troublesome step for mechanically wiping off the black powder as by blushing after stud welding, and this would lead to an elevated cost and the disadvantage that rust is still inevitably formed to a degree, at the weld zone, even if a suitable chemical treatment is made.